They don't have true competition, what they lose out on is market share with hyperscalers, since OpenAI would have no plans to share inference hardware with any other company right now. Plus, I don't know how does NVIDIA's investment equation pans out long terms given OpenAI will be investing in more purpose built inference stack for the future.

they're still kings for training, though I've heard Anthropic is training now on JAX+TPU setup, so might not be a monopoly in that segment.